Primers backing out is a tell tale sign of wear at the locking block and the corresponding barrel recess.

WOOT WOOT!!!

Seems my barely "educated guess" might be close enuff that I won't feel foolish if it turns out to be somethin' different!
If Zipper described close to my guess... and he has far more gun hours than I do I bet... than I feel exalted and takin' my grinnin' mug to the kitchen for an instant "cap-a-cheeno"... cuz I feel smart now!